1. Field of the Invention
This invention relates to an inverter suitable for use with portable AC power supply units.
2. Description of the Related Art
Inverters have been incorporated in portable AC power supply units, drivers of AC motors, uninterruptable power supply units, etc. As one of modes of application, a plurality of, for example, two portable AC power supply units are connected in parallel with each other to drive a load. In this case, the portable AC power supply units are operated with output frequencies in synchronism with each other. Current (cross current) flows from one of the power supply units to the other when a change in the load etc. results in a subtle change in the frequency of either power supply unit. The cross current sometimes breaks circuit components of the portable AC power supply unit. The cross current flows from the power supply unit with a higher output frequency to the one with a lower output frequency.
As one of measures preventing the cross current flowing between the portable AC power supply units, a lag or lead in a phase between output voltage and output current is monitored. An output frequency is adjusted on the basis of the phase lag or lead, whereby the cross current is prevented. FIG. 22 shows one of the conventional arrangements for preventing the cross current. A portable AC power supply unit 1 comprises an engine drive AC generator 2 and an inverter unit 3 having output terminals 3a and 3b from which a sinusoidal AC voltage is delivered. The inverter unit 3 includes a rectifier circuit 4 rectifying a three-phase AC voltage generated by the AC generator 2, a smoothing capacitor 5, a single-phase full-bridge inverter circuit 6, a filter circuit 7, a control circuit 8 and a drive circuit 9. The control circuit 8 includes a microcomputer 10 and a pulse-width-modulation (PWM) circuit 11 generating a drive signal. The inverter circuit 6 is adapted to be connected to a load. When a plurality of the portable AC power supply units 1 are to be connected to each other, the inverter circuits 6 of the respective power supply units 1 are connected to each other.
In the above-described arrangement, the control circuit 8 controls the generator 2 so that the engine maintains a predetermined number of revolution. The control circuit 8 further performs a PWM control so that a sine wave AC voltage having a predetermined voltage (for example, an effective value of 100V) at a predetermined frequency (50 Hz or 60 Hz) is delivered from the output terminals 3a and 3b. The control circuit 8 further includes an output voltage detecting circuit 12 detecting an output voltage of the inverter circuit 6, an output current detecting circuit 13 detecting an output current of the inverter circuit 6 and a phase difference detecting circuit 14 detecting a phase difference between the output voltage and the output current. The control circuit 8 increases an output frequency when the output current lags behind the output voltage. Further, the control circuit 8 decreases the output frequency when the output current leads the output voltage. Consequently, an output balance is achieved between the two AC power supply units 1 connected in parallel with each other. The output frequencies are adjusted in a range between 49.90 Hz and 50.10 Hz when the portable AC power supply units are of 50 Hz specification.
In the above-mentioned detection of the phase difference, the phase difference is detected by measuring a time from a zero cross point of the output voltage (AC) to a zero cross point of the output current. However, when a detected waveform of the output current is distorted, the zero crossing may occur twice or the timing of the zero crossing may not be normal. As a result, the output currents are not balanced between the power supply units in a parallel operation of the power supply units.
Therefore, an object of the present invention is to provide an inverter in which the output currents are balanced in the parallel operation of the power supply units.
The present invention provides an inverter comprising a DC power supply circuit, an inverter circuit having a plurality of switching elements and switching an output of the DC power supply circuit on the basis of a PWM signal to deliver a high-frequency voltage, a filter circuit converting the high-frequency voltage to a substantially sinusoidal AC voltage, a power detector detecting an effective or wattless power of the AC power, a phase angle calculator calculating a phase angle of current relative to voltage from the detected effective or wattless power, a phase. detector detecting a leading or lagging state of the phase angle, and a controller decreasing a frequency of the output voltage when the phase detector detects the leading state of the phase angle, the controller increasing the frequency of the output voltage when the phase detector detects the lagging state of the phase angle.
According to the above-described arrangement, the effective power or wattless power contains a phase angle element between the output voltage and the output current. The effective or wattless power is detected, and the phase angle calculator calculates a phase angle on the basis of the detected effective or wattless power. Consequently, an accurate phase angle can be detected even when the output current or the output current detector contains a waveform distortion and accordingly, the accuracy in the detection of the phase angle can be improved. Furthermore, the phase detector detects the leading or lagging state of the calculated phase angle, and the controller decreases the frequency of the output voltage when the phase detector detects the leading state of the phase angle. The phase detector increases the frequency of the output voltage when the phase detector detects the lagging state of the phase angle. Consequently, an accurate frequency control can be carried out, and the output currents are balanced in the parallel operation of the power supply units.